A microbial community originating from brewery waste produced methane, acetate, and hydrogen when selected on a granular graphite cathode poised at −590 mV versus the standard hydrogen electrode (SHE) with CO ₂ as the only carbon source. This is the first report on the simultaneous electrosynthesis of these commodity chemicals and the first description of electroacetogenesis by a microbial community. Deep sequencing of the active community 16S rRNA revealed a dynamic microbial community composed of an invariant Archaea population of Methanobacterium spp. and a shifting Bacteria population. Acetobacterium spp. were the most abundant Bacteria on the cathode when acetogenesis dominated. Methane was generally the dominant product with rates increasing from 4 mM day ⁻¹ (accumulated to 28.5 mM over 12 days), and methanogenesis ceased following the addition of 2-bromoethanesulfonic acid. Traces of hydrogen accumulated during initial selection and subsequently accelerated to >11 mM day ⁻¹ (versus 0.045 mM day ⁻¹ abiotic production). The hypothesis of electrosynthetic biocatalysis occurring at the microbe-electrode interface was supported by a catalytic wave (midpoint potential of −460 mV versus SHE) in cyclic voltammetry scans of the biocathode, the lack of redox active components in the medium, and the generation of comparatively high amounts of products (even after medium exchange). In addition, the volumetric production rates of these three commodity chemicals are marked improvements for electrosynthesis, advancing the process toward economic feasibility.

Bacterial populations vary in their stress tolerance and population structure depending upon whether growth occurs in well-mixed or structured environments. We hypothesized that evolution in biofilms would generate greater genetic diversity than well-mixed environments and lead to different pathways of antibiotic resistance. We used experimental evolution and whole genome sequencing to test how the biofilm lifestyle influenced the rate, genetic mechanisms, and pleiotropic effects of resistance to ciprofloxacin in Acinetobacter baumannii populations. Both evolutionary dynamics and the identities of mutations differed between lifestyle. Planktonic populations experienced selective sweeps of mutations including the primary topoisomerase drug targets, whereas biofilm-adapted populations acquired mutations in regulators of efflux pumps. An overall trade-off between fitness and resistance level emerged, wherein biofilm-adapted clones were less resistant than planktonic but more fit in the absence of drug. However, biofilm populations developed collateral sensitivity to cephalosporins, demonstrating the clinical relevance of lifestyle on the evolution of resistance.

Microbial fuel cells (MFCs) are bioelectrochemical devices capable of converting chemical energy to electrical energy using bacteria as the catalysts. Mechanisms of microbial electron transfer to solid electrode surfaces are not well defined in most electrochemically-active microorganisms, particularly for Gram-positive bacteria. In this study, we investigated the electrochemical characteristics of the Gram-positive, thermophilic bacterium Thermincola ferriacetica strain Z-0001. This organism was capable of transferring electrons from acetate to the anode of an MFC to generate an electric current. T. ferriacetica exhibited rapid recovery of current following medium exchanges, recovering to near-maximum current output in less than three hours. The recovery of electrons from acetate was 97% in air-cathode MFCs inoculated with T. ferriacetica. Further insights into the anode reduction by these biofilms were gained through cyclic voltammetry (CV). A continuous steady-state current was reached above −0.1 V vs.Ag/AgCl reference electrode in CV scans of an established T. ferriacetica biofilm. A catalytic wave with a midpoint potential consistently near −0.28 V indicated a continuous electron-transporting interface between the attached microbial biofilm and the electrode surface. Additionally, no significant peaks were observed when scanning cell-free spent medium from active MFCs. These data suggest that T. ferriacetica directly transfers electrons to an electrode through a mechanism that is tightly associated with the biofilm that forms on the electrode. This is the first mechanistic insight into how Gram-positive extracellular electron transfer might occur without the addition of soluble electron shuttling mediators. These mechanistic evaluations will be essential for the improvement and application of such biocatalysts in microbial fuel cells and other bioelectrochemical systems.

Production of hydrogen and organic compounds by an electrosynthetic microbiome using electrodes and carbon dioxide as sole electron donor and carbon source, respectively, was examined after exposure to acidic pH (∼5). Hydrogen production by biocathodes poised at −600 mV vs. SHE increased>100-fold and acetate production ceased at acidic pH, but ∼5–15 mM (catholyte volume)/day acetate and>1,000 mM/day hydrogen were attained at pH ∼6.5 following repeated exposure to acidic pH. Cyclic voltammetry revealed a 250 mV decrease in hydrogen overpotential and a maximum current density of 12.2 mA/cm² at −765 mV (0.065 mA/cm² sterile control at −800 mV) by the Acetobacterium-dominated community. Supplying −800 mV to the microbiome after repeated exposure to acidic pH resulted in up to 2.6 kg/m³/day hydrogen (≈2.6 gallons gasoline equivalent), 0.7 kg/m³/day formate, and 3.1 kg/m³/day acetate ( = 4.7 kg CO₂ captured).